The judge notes two problems (at least!):
(1.) the FTC approved the purchase of Instagram and WhatsApp. Why does it want to change its mind?
(2.) they don’t even attempt a serious market definition. If Facebook is a monopoly, what is it monopolizing? Who else is in the market? This is an absolutely central question in any monopolization or horizontal merger case. Indeed, getting the judge to accept your theory of the market is a big part of the job.
The economics staff at the FTC is not incompetent (I know several people there), but they have a tough job in this case. If it has been up to me I would not have allowed the Instagram and WhatsApp purchases, but the FTC itself approved them!
Perhaps they did know the case wouldn't have a chance. Perhaps there isn't a plan to follow up with the actual complaint. It's not like the federal government has seriously pursued anti-trust activity in recent decades.